The role of melting temperature and electron-phonon coupling in the formation of clustered vacancy defects from heavy-ion generated displacement cascades

The production of clustered vacancy defects (dislocation loops and stacking fault tetrahedra) from heavy-ion-generated displacement cascades has been investigated by transmission electron microscopy in a series of Cu-Ni and Ag-Pd alloys. The density of defects decreases as the solute content of the alloy increases, but not in a simple manner. These results are interpreted in terms of changes in the lifetime of the thermal spike or molten zone generated within the cascade. The factors which have been considered to affect the lifetime are the melting temperature, the degree of coupling between the electron and phonon systems, and the influence of solutes on the character of the molten zone. A comparison with stacking-fault energy data has also been made. It is demonstrated that the results in the Ag-Pd correlate approximately with changes in the alloy melting temperature and stacking-fault energy, whereas in Cu-Ni the results correlate with the change in the strength of electron-phonon coupling. Further tests of these models are made by examining previously published data from Cu alloys and Ni-Cr alloys. The Cu alloys are not affected by electron-phonon coupling and the changes observed appear to reflect the changes in melting temperature and solute effects. In the Ni-Cr system the density of states is large and constant and the decrease in defect yield appears to reflect the change in the strength of electron-photon coupling as solute is added. (Author)